1. Field of the Invention
The present invention relates to a pattern transfer method and a transfer apparatus which are usable, for example, for lithography to fabricate semiconductor integrated circuits. More particularly, the present invention relates to a technique whereby a pattern on a mask is transferred onto a radiation-sensitive substrate by using a charged particle beam, e.g., an electron beam, an ion beam, etc.
2. Description of the Related Art
In a pattern transfer process that uses a charged particle beam, if the beam current is large, blur caused by the Coulomb interaction effect is introduced into a pattern image projected onto a radiation-sensitive substrate (e.g., a wafer as a substrate of a semiconductor integrated circuit). The greater part of the blur caused by the Coulomb interaction effect can be corrected by re-adjusting the focus position of the pattern projection lens system. However, a part of the blur caused by the Coulomb interaction effect remains uncorrected. There is a variable shaped beam type exposure system in which the cross-sectional configuration of the charged particle beam is varied within a range of about 10 by 10 micrometers square to carry out pattern transfer. In this type of exposure system, blur caused by the Coulomb interaction effect is predicted from the area of the shaped beam and system parameters (i.e., the beam current, the beam aperture angle, the beam acceleration voltage, and the optical length of the lens system), and the focus of the lens system is adjusted according to the result of the prediction. Focus adjustment made for the purpose of eliminating blur caused by the Coulomb interaction effect may be particularly called "refocusing".
Incidentally, there is a so-called divide transfer type system in which a part or all of a plurality of small areas provided on a mask are sequentially irradiated with a charged particle beam to transfer images of patterns provided in the irradiated small areas onto predetermined positions on a radiation-sensitive substrate. In this type of system, the irradiation range on the radiation-sensitive substrate by a single shot of charged particle beam is from 100 by 100 to 1,000 by 1,000 micrometers square; this is considerably wider than that in the variable shaped beam type system. It has recently been reported that blur caused by the Coulomb interaction effect is small when the irradiation range is so wide as stated above (Particle-Particle Interaction Effects in Image Projection Lithography; S. D. Berger et al; J. Vac. Sci. Technol. B11(6). November/December 1993 p.2294-). This is considered to be a great advantage of the divide transfer type system. That is, when the amount of blur caused by the Coulomb interaction effect is restricted within a predetermined value, the divide transfer type system allows use of a larger beam current than in the case of the variable shaped beam type, thus enabling an improvement in the throughput of the transfer process.
However, the divide transfer type system also suffers from some problems. That is, patterns (equivalent to charged particle beam transmitting portions) are not always uniformly distributed over each individual small area on the mask; patterns are distributed in various forms. In a case where patterns particularly concentrate on a limited narrow area in a small area, when pattern transfer is carried out for that small area, the charged particle beam irradiation range on the radiation-sensitive substrate practically narrows, thus increasing blur caused by Coulomb interaction effect. For example, let us consider a case where, as shown in FIGS. 5(a) and 5(b) of the accompanying drawings, small areas A and B which are provided on a mask are each provided with the same number of charged particle beam transmitting patterns PT (hatched portions in the figures) which are equal in both shape and area to each other. In this case, the total areas of the patterns PT in the small areas A and B are the same. However, in the small area B, shown in FIG. 5(b), the charged particle beam concentrates on a narrower area than in the small area A, shown in FIG. 5(a). Accordingly, blur caused by the Coulomb interaction effect is greater in the small area B than in the small area A. Therefore, blur caused by the Coulomb interaction effect cannot be accurately corrected simply by effecting focus adjustment according to the pattern density (i.e., the total area of transmitting patterns/the area of the small area) for each small area.